The present invention relates to an apparatus for and a method of shape checking patterns and, more particularly, to an apparatus for and a method of shape checking patterns requiring highly accurate pattern images such as mask patterns.
The present invention also relates to light-permeable area shape checking apparatuses and, more particularly, to a transmitting area shape checking apparatus for shape checking light-permeable areas of a mask pattern or the like by illumination thereof. The present invention may be suitably used for checking the edge shapes of, for instance, a reticule used in a photographic process in IC manufacture. Of course the present invention is suitably applicable to the shape check of not only reticules but also mask patterns for masking photo-sensitive resin or the like in a photo-etching technique in IC manufacture. Also, it is possible to apply the present invention not only to the IC manufacture but also to the shape check of mask members used for masking against light transmission. Furthermore, the present invention is suitably applicable not only to mask members but also to objects which can be checked by checking the light permeability.
Japanese Patent Disclosure No. 63-567021 shows an apparatus for and a method of shape checking patterns of the pertaining type. According to this disclosure, a pattern is illuminated to obtain transmitted light therefrom with CCD or like sensor, and data obtained therefrom is checked by comparison as to whether it has a desired pattern.
In the shape check of a photo-mask used for drawing an integrated circuit pattern which requires super-high shape accuracy, two-dimensional deviations and distortions which arise between and in the photo-mask pattern and the obtained pattern data due to various causes pose problems.
There were techniques seeking to meet the required accuracy by measuring beforehand deviations and distortions generated in pattern data, correcting the deviations and distortions according to the result of measurement and making the pattern comparison with the corrected pattern data.
The above prior art pattern shape check apparatus had a problem that variations of light from the pattern, variations of the incident light quantity and secular variations of detecting means reduce the pattern shape recognition accuracy, thus reducing the sharpness of image. To solve this problem, the applicant earlier developed a technique disclosed in Japanese Patent Application No. 9-357846, which will now be described.
Like the technique described above, a pattern is illuminated, but the illuminating light is partly used for the level detection thereof. The individual branch light beams are detected by respective CCD sensors or the like. These signals contain components representing illuminating light level variations. For correcting these signals, the branched light for the level detection is amplified to the same level as the detected level of directly transmitted illuminating light, in the light obtained from the pattern, and the amplified light is used to divide the light obtained from the pattern.
More specifically, the illuminated light level variations to the pattern in dependence on the incidence position are normalized (to a constant level) by using the amplified light which includes the level variations to correct the illuminated light level variations. It is thus possible to make utmost use of the illuminated light level range detectable by the sensor although the illuminating light level may be reduced.
This technique, as will be described later in the description of the embodiment, permits removal of noise at zero level and maximum level of the signal by A/D converting the normalized signal after providing offsets for an upper and a lower portions of the signal. Even in this case, however, a commonly termed edge portion of signal between the detected levels of zero and maximum contains noise. This noise is dependent on the level of light transmitted through the pattern before the correction. Therefore, the noise level is varied whenever the light level is varied. For this reason, the pattern shape recognition accuracy is reduced with reducing illuminating light level, which is a first problem of the prior art technique.
In summary, the problem posed in conventional pattern shape recognition is that variations of light from a pattern to be checked, illuminating light level variations and variations of detecting means in long use results in reduced accuracy of the pattern shape recognition and reduced sharpness of image.
Heretofore, the shape check of an object is made by comparing an image of the object with a reference image. The reference image is obtained by picking up the good sample of the subject or generated from CAD data or the like. The two images are compared after binalizing them. For example, Japanese Patent Disclosure No. 5-46747 discloses a method of obtaining binalized images of wiring patterns of printed circuit boards.
However, edge areas of an object can not be satisfactorily checked by comparing two images. In the meantime, by comparing an image of an object and the reference image as multiple-value data such as 256 gradation data, satisfactory comparison of the rising falling state of edge areas can be obtained, thus permitting satisfactory shape check.
In a different method of shape check, a CPU or a program is used. In this case, light emitted from a light source and transmitted through an object is photoelectrically converted in a CCD sensor or the like. In this case, the capacity of the light source such as a laser and the sensor is subject to changes in long use. After long use, therefore, it is no longer possible to obtain satisfactory comparison of the object image with the reference image. To evade influence of the secular changes in the capacity of the laser, the laser beam is split, and the ratio or the difference between light transmitted through an object and light not transmitted therethrough is calculated.
However, even by splitting the laser beam and calculating the ratio between the light transmitted through an object and the light not transmitted therethrough, the measurement errors due to the sensor and associated analog circuit can not be removed. Neither it is impossible to remove influence of noise generated in the sensor and following analog circuit.
In the case of checking the edge areas from gradation changes in the rising and falling, the image gradation is desirably not changed with pattern areas free from changes.
However, changes in the image gradation occurs even with pattern areas free from changes due to shot noise in the sensor, measurement errors in the A/D converter and noise in the following analog circuit. These changes result in accuracy reduction in the comparison of the object image with reference image or like process.
More specifically, the prior art arrangement with the setting of the level of light transmitted through the pattern to be the maximum level and the level of light not transmitted through the pattern to be the minimum level, has a drawback that the maximum and minimum level areas are unstable, which is a second problem of the prior art technique.